Clustering refers to configuring entities having a similarity among a number of entities of a group called a cluster. Therefore, a level indicating similarity among respective entities is needed in order to configure a cluster and a similarity between two entities i and j is generally expressed as a distance (i, j). However, in a packet timing standard such as current packet time protocol (PTP) or network time protocol (NTP), only a method for synchronization between master and slave is described, but a method for configuring a cluster between master and slave is not described.
When synchronizing phase and frequency in a small cell including a femtocell by using a network assisted global navigation satellite system (AGNSS), the intensity of a signal transmitted from the AGNSS may be different according to the environment where the small cell is located. It may not be possible to secure a line of sight (LOS) in severe cases. If the intensity of the signal is low the accuracy of synchronized time may also become low. Another problem is that when a small cell is located in indoors, the possibility of increasing the accuracy becomes low.
When synchronizing phase and frequency in the small cell by using a timing packet, packet delay variation (PDV) is largely increased according to queuing delay, network overload, network congestion, temporary network blocking or the like caused by router and switch based on a backhaul network connected to the small cell, and this reduces phase and frequency synchronization quality of the slave or fails to establish synchronization in severe cases.
In the conventional master-slave structures using a timing packet, the roles of the maser and slave are fixed. This causes a problem since the number of slaves is limited depending on the capacity of the master. The PDV is also increased due to consideration of the master's location and timing packet convergence as multiple slaves converge on one master.
In a fixed type of master-slave structure, the distance over a network between the master and slave changes. The measure of dispersion for this is large (the network distance is increased when the number of routers or switches becomes large). This makes it difficult for a slave having a large network distance to be synchronized with a master in phase and frequency.
When using a synchronizing method using a pilot channel and a synchronization channel of a neighbor cell (macro cell or small cell), since the neighbor cell should be within a certain distance to receive a radio wave that can reach the neighbor cell, there is a problem in that a synchronization reference cannot be obtained due to the absence of a neighbor cell when a small cell is positioned to improve the shadow area for coverage expansion.